As a protection against fraud, it is known to incorporate diphenylguanidine, preferably complexed with various agents, as colorless ink in valuable papers that will develop a conspicuous color for example during attempted erasure by bleaching cheques.
Diphenyl-guanidine will indeed provide a conspicuous stain in the case ink eradicators such as oxidising agents (typically sodium hypochlorite) are used during an attempt to remove the dye and/or the pigments which have been sprayed or sprinkled on the valuables.
In the above prior art the inks were used for safety papers at the manufacturing or printing steps.
However, there is a need for a colored ink formulation stable for a long period of time and within a large range of temperature which produces stable and permanent stain that cannot be leached by current agents such as sodium hypochlorite.